There are spaces between particles. Imagine you put pebbles into a measuring cylinder. The reading is 200mL. Then you get 200mL of sand, and mix it in. The result is not 400mL, as there are spaces between the pebbles, and the sand can fill them up.

So, if you mix equal volumes of alcohol and water, the result is not twice the volume of alcohol or water, but slightly less. See the diagram on the right for what the particles look like. Or, if you put sugar into coffee, the coffee level does not rise up the same volume as the sugar. This is because there are spaces between particles.

Particles are always moving, although they do move differently - this will be explained in the states of matter chapter later on. For example, when water is boiled, the 'steam', which are actually tiny water particles, cannot stay in one place. More about this will be discussed in the next chapter, which is about diffusion.